This book presents essential information on the use of the immunodeficient C.B.-17 SCID/SCID mouse for studying human hematopoiesis in vivo. Because of the lack of both humoral and cellular immunity, this mouse can be a unique host for implantation of human hematopoietic tissue using different methods. In the first section, in vivo development of human hematopoietic stem cells and differentiation of human T cells are described. In addition, the effects of growth factors and toxic agents of human hematopoiesis are described. The second section contains chapters in which the human immune responses in the SCID mouse are reviewed. The third section covers SCID mouse models to study human infectious diseases, leukemias and genetic disorders.